1. Field of the Invention
This invention relates to the stabilization of dicalcium phosphate dihydrate. More particularly, this invention relates to the stabilization of dicalcium phosphate dihydrate employing a tetra-alkali metal pyrophosphate.
2. Description of the Prior Art
Dicalcium phosphate dihydrate, i.e., CaHPO.sub.4.2H.sub.2 O, is commonly used as a polishing agent in dentifrice compositions. In many cases the compositions also contain other ingredients which cause the dicalcium phosphate dihydrate present therein to dehydrate to the anhydrous form and/or to hydrolyze, yielding acidic products. The dehydration and/or hydrolysis causes undesirable conditions to occur in dentifrice compositions. One undesirable condition which has been attributed to such dehydration and/or hydrolysis is a change in composition consistency resulting in phase separation, thickening or, in extreme cases, solidification. Another problem which has occurred is tube corrosion which has resulted in tube blackening. In severe cases of corrosion, the tube is pitted and/or gas generated which exerts sufficient pressure to cause tube bulging.
It has therefore been customary to stabilize the dicalcium phosphate dihydrate employed in dentifrice compositions. Reference herein to stabilization of dicalcium phosphate dihydrate means that the tendency of the dicalcium phosphate dihydrate to dehydrate to the anhydrous form and/or to hydrolyze as previously described is reduced, thus making it more suitable for use in certain dentifrice compositions. Stabilization has usually been achieved by the addition of such materials as pyrophosphoric acid, sodium pyrophosphate or sodium-calcium pyrophosphate to a slurry of dicalcium phosphate dihydrate in water. The slurry has then been filtered and the calcium phosphate dihydrate is recovered for use in dentifrice compositions. Various other materials have been proposed for stabilization of dicalcium phosphate dihydrate, for example, it has been proposed to dry mix calcium phosphate with trimagnesium orthophosphate.
A process for preparing dicalcium phosphate dihydrate stabilized with a tetra-alkali metal pyrophosphate is disclosed in U.S. Pat. No. 2,287,699 issued to H. V. Moss et al on June 23, 1942. According to the procedure described therein, a dicalcium phosphate dihydrate is prepared by the addition of lime slurry to phosphoric acid under conditions such that the dicalcium phosphate dihydrated compound is precipitated. After precipitation is complete, an alkali metal pyrophosphate is added to the mother liquor containing the precipitate and the resulting slurry is heated to a temperature somewhat above the precipitation temperature and maintained at the elevated temperature for a short period of time during which the pH of the mother liquor is maintained above a pH of 7 by incremental additions of a tetra-alkali metal pyrophosphate. The stabilized dicalcium phosphate dihydrate is recovered by filtration, dried and ground.
In U.S. Pat. No. 3,012,852 issued to G. D. Nelson on Dec. 12, 1961, there is disclosed a process for preparing stabilized dicalcium phosphate dihydrate, which process involves the addition of a minor amount of pyrophosphate ions to phosphoric acid which is then employed in a reaction with a calcium source such as calcium hydroxide to form the dicalcium phosphate dihydrate.
Another process for stabilizing dicalcium phosphate dihydrate is described in U.S. Pat. Nos. 3,066,056 and 3,169,096 issued to Julian R. Schlaeger et al on Nov. 27, 1962 and Feb. 9, 1965, respectively. The process described therein employs pyrophosphoric acid and calcium-sodium pyrophosphate. In accordance with that process, pyrophosphoric acid is added to a reaction mixture resulting from the reaction of orthophosphoric acid with milk of lime and having a pH between about 5 and the neutralization point. The mixture is then neutralized with the addition of milk of lime, and the dicalcium phosphate dihydrate is removed, dried and admixed with finely divided calcium-sodium pyrophosphate to produce the stabilized dicalcium phosphate dihydrate product. It is stated therein that both the pyrophosphoric acid and calcium-sodium pyrophosphate additions are essential since neither alone produces a dicalcium phosphate dihydrate composition having desired stability.
In U.S. Pat. No. 3,411,873 issued to H. Harnisch et al on Nov. 19, 1968 there is described a process for stabilization of dicalcium phosphate dihydrate which employs a magnesium-ammonium phosphate as a stabilizer, preferably in combination with an alkali metal or alkaline earth metal pyrophosphate. It is stated therein that the stabilizers can readily be introduced into the reactor during the production of the dicalcium phosphate but before isolation thereof, at a pH value between 7.5 and 8.5.
Although the prior art processes are effective in the stabilization of dicalcium phosphate dihydrate, such processes have certain deficiencies. For example, substantial losses of pyrophosphate have been associated with processes for stabilization by the addition of tetra-alkali metal pyrophosphate to an aqueous mixture containing dicalcium phosphate dihydrate. Thus, to achieve a desired stabilizing level of pyrophosphate in the dicalcium phosphate dihydrate, it has been required to employ much higher levels of pyrophosphate in the form of the tetra-alkali metal pyrophosphate in such processes. Other processes, such as dry mixing dicalcium phosphate dihydrate with certain stabilizing materials, are not very effective in achieving a desired degree of stabilization. Thus, it can be seen that there is a need for an improved process for the stabilization of dicalcium phosphate dihydrate, which process results in an efficient utilization of the stabilizing agent.
It is therefore an object of the present invention to provide an improved process for the stabilization of dicalcium phosphate dihydrate employing a tetra-alkali metal pyrophosphate as a stabilizer. These and other objects will become apparent from the following detailed description.